The invention pertains to a high reliability signal sensing circuit particularly suited for use as an electronic flame sensing circuit forming a part of a burner control apparatus. The invention is described with specific reference to such a flame sensing use, but other applications for use of the invention undoubtedly exist as well. Generally, a flame sensing circuit includes a sensor element physically located close to the site of the flame so as to provide a sensor signal having a predetermined level or operating state when a flame is present and some other level or operating state when flame is not present. The signal supplied directly by the sensor may by its level indicate the level of ultraviolet or infrared radiation produced by the flame when present, or may directly sense presence of the hot flame gasses. Typically, there is a processing or detector circuit which receives the sensor signal, which is usually in an analog format, and converts it into a signal which has a form usable by the burner control or other apparatus basing its operation on the sensor signal. The processing or detector circuit may be located relatively close to the flame, and connected to the flame sensor.
One typical type of flame sensor is generally referred to as a flame rod, and uses the inherent capability of the ionized particles created by a flame to conduct current between conductors placed in the flame. In a preferred embodiment of such a device, the two conductors are the flame rod conductor of relatively small area and the burner itself of relatively large area. The difference in areas creates a rectifier effect in the conduction of an AC voltage placed between the flame rod and the burner. Because the burner is relatively larger than the flame rod, the rectifier formed by them produces a negative DC voltage. There is no theoretical reason why the burner cannot be physically smaller than the flame rod, but practical considerations dictate the opposite, so that the flame current generated by a flame rod may be considered for the discussion following as negative. However, the principles of the invention is equally applicable for a positive sensor signal.
It is important that such sensors and the circuits with which they operate be extremely reliable in detecting presence of flame, since whenever flame is not present it is of paramount importance that fuel flow to the burner be immediately stopped, and in the case of a pilot flame, not be started. It is of course inconvenient if the sensing circuit indicates absence of a flame when in fact there is flame present, because this results in emergency shutdown of fuel flow to the burner. However, this condition does not create any serious safety hazard.
There have heretofore been a variety of designs which have been used which attempt to immunize the sensors and sensing circuits against failures of all sorts whose effect is to simulate presence of a flame which is actually not present. Some approaches include redundant signal paths or redundant components. Others use frequent brief tests of the sensor and/or sensing circuit which identify faulty operation of the sensor or circuit very quickly after the fault occurs. Some test the circuit each time during the burner startup sequence. Such tests may be done for example by injecting a simulated sensor signal into the circuit.
However, certain types of failures in the flame sensing circuitry can closely mimic the signal normally provided by the sensor in response to presence of flame. This situation can arise for example, where a voltage normally present on the detector's circuit board leaks into the signal path and simulates a signal level indicative of presence of flame. While frequent testing can detect many of these failures, it is difficult to completely avoid the potential for a certain number of such events to cause improper indication of presence of flame. In the situation where the circuit is attempting to detect flames, even one failure to properly do so is too many.
Accordingly, the safety of burner control systems and other safety critical systems can be improved by reducing or eliminating the possibility of leakage currents which simulate actual sensor signal levels indicative of flame.